Different methods have proved to be especially effective for coding audio signals. Thus what is known as the CELP (Code Excited Linear Prediction) technology has proved especially useful for example for high-quality coding of voice signals which exhibit a good quality and with simultaneously low bit rates of the coded data stream. CELP operates in the time domain and is based on an excitation model for a variable filter. In this case the voice signal is represented both by filter parameters and also by parameters which describe the excitation signal.
The appropriate decoders are generally mentioned in relation to coders, with said decoders being able to decrypt or decode the coded data. The corresponding communication devices feature what is known as a codec to enable them to transmit and receive data which is required for communication.
For coding of music and voice signals which are to exhibit a very high quality especially at higher bit rates of the coded data stream, above all perceptual codecs (codec=coder/decoder) have become established. These perceptual codecs are based on a reduction of information in the frequency range and they utilize masking effects of the human hearing system, i.e. for example the fact that specific frequencies or changes that a human being cannot perceive are also not represented. This reduces the complexity of the coder or codec. Since these coders mostly operate with a transformation of the time signal in the frequency domain, in which case the transformation is undertaken for example using MDCT (Modified Discrete Cosine Transformation), these devices are also often referred to as transform coders or codecs. This term will be used within the context of this patent application.
In recent times what are known as scalable codecs have increasingly come into use. Scalable codecs are codecs which generate an excellent audio quality at a relatively high bit rate of the coded data stream. This produces relatively long packets to be transmitted periodically.
A packet is a plurality of data which arises within a period of time and which can also be transmitted together in this packet. Often important data is transmitted first in packets and less important data is transmitted later. The option exists however with these long packets of shortening the packet by removing part of the data, especially by truncating the part of the packet transmitted latest in time. This naturally brings with it a deterioration in quality.
Because of the characteristics previously mentioned it is best for scalable codecs to operate at low bit rates with CELP codecs and at higher bit rates with transform codecs. This has led to the development of hybrid CELP/transform codecs which code a basic signal with good quality according to the CELP method and additionally generate a supplementary signal according to the transform codec method with which the basic signal is improved. This then results in the desired excellent quality.